1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for feedback transmission in a wireless communication system employing a relay station.
2. Related Art
The institute of electrical and electronics engineers (IEEE) 802.16e standard was adopted in 2007 as a sixth standard for international mobile telecommunication (IMT)-2000 in the name of ‘WMAN-OFDMA TDD’ by the ITU-radio communication sector (ITU-R) which is one of sectors of the international telecommunication union (ITU). An IMT-advanced system has been prepared by the ITU-R as a next generation (i.e., 4th generation) mobile communication standard following the IMT-2000. It was determined by the IEEE 802.16 working group (WG) to conduct the 802.16m project for the purpose of creating an amendment standard of the existing IEEE 802.16e as a standard for the IMT-advanced system. As can be seen in the purpose above, the 802.16m standard has two aspects, that is, continuity from the past (i.e., the amendment of the existing 802.16e standard) and continuity to the future (i.e., the standard for the next generation IMT-advanced system). Therefore, the 802.16m standard needs to satisfy all requirements for the IMT-advanced system while maintaining compatibility with a mobile WiMAX system conforming to the 802.16e standard.
Effective transmission/reception methods and utilizations have been proposed for a broadband wireless communication system to maximize efficiency of radio resources. An orthogonal frequency division multiplexing (OFDM) system capable of reducing inter-symbol interference (ISI) with a low complexity is taken into consideration as one of next generation wireless communication systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols, and is then transmitted by being carried on each of separated N subcarriers. The subcarriers maintain orthogonality in a frequency dimension. Each orthogonal channel experiences mutually independent frequency selective fading, and an interval of a transmitted symbol is increased, thereby minimizing inter-symbol interference.
When a system uses the OFDM as a modulation scheme, orthogonal frequency division multiple access (OFDMA) is a multiple access scheme in which multiple access is achieved by independently providing some of available subcarriers to a plurality of users. In the OFDMA, frequency resources (i.e., subcarriers) are provided to the respective users, and the respective frequency resources do not overlap with one another in general since they are independently provided to the plurality of users. Consequently, the frequency resources are allocated to the respective users in a mutually exclusive manner. In an OFDMA system, frequency diversity for multiple users can be obtained by using frequency selective scheduling, and subcarriers can be allocated variously according to a permutation rule for the subcarriers. In addition, a spatial multiplexing scheme using multiple antennas can be used to increase efficiency of a spatial domain.
A user equipment (UE) may transmit a feedback through uplink. The feedback may include channel information necessary for data transmission. A base station (BS) may schedule radio resources using the feedback received from the UE and transmit data. A closed loop scheme is a scheme of transmitting data by compensating for channel information included in a feedback from UE, and an open loop scheme is a scheme of transmitting data by not compensating for channel information included in a feedback from UE. A feedback may not be transmitted according to the open loop scheme, and a BS may do not use channel information included in a feedback although the channel information is included in the feedback. In general, in a wireless communication system, the open loop scheme may be applied to a channel environment for UE that moves at high speed, and the closed loop scheme may be applied to a channel environment for UE that moves at low speed. The open loop scheme is applied to a channel for UE that moves at high speed because the channel is greatly changed, making channel information, included in a feedback, difficult to be reliable. The closed loop scheme may be applied to a channel environment for UE that moves at low speed because the channel environment is relatively less changed and channel information included in a feedback is reliable and less sensitive to delay.
A wireless communication system including a relay station (RS) has recently been developed. The RS is employed for cell coverage extension and transmission capability improvement. A BS provides a service to an MS located in a coverage boundary of the BS via the RS, and thus can obtain an effect of extending the cell coverage. In addition, the RS improves signal transmission reliability between the BS and the MS, thereby improving transmission capacity. The RS can be used when the MS is located in a shadow area even if the MS is located within the coverage of the BS.
In a wireless communication system employing a relay station, the relay station can also transmit an uplink feedback to a base station. Meanwhile, since the base station and the relay station have fixed positions in general, there is a high possibility that link quality between the base station and the relay station is better than link quality between the base station and a user equipment. Therefore, as an uplink feedback transmission method of the relay station, a new method different from the conventional uplink feedback transmission method of the user equipment can be introduced.
There is a need to propose a method for effective uplink feedback transmission of the relay station.